Polyether polyols are useful flexible segments for polyurethanes, thermoplastic elastomers, and unsaturated polyester resins. Commercial polyethers such as poly(propylene oxide), poly(ethylene oxide), and poly(tetrahydrofuran) are commonly made by ring-opening polymerization of cyclic ethers. Poly(tetrahydrofuran) and poly(ethylene oxide) have primary hydroxyl end groups, which offer high reactivity with polyisocyanates in urethane reactions and fast esterification in making unsaturated polyester resins and thermoplastic polyesters. Poly(tetrahydrofuran) and poly(ethylene oxide) are also crystalline and solids at room temperature. Non-crystalline, liquid polyethers are often desirable for use in coatings, sealants, adhesives, and elastomers because they are easily handled and can offer flexibility advantages over crystalline polyethers. Poly(ethylene oxide) is water soluble, and elastomers or coatings made from it typically have poor water resistance. Poly(propylene oxide) is non-crystalline and more hydrophobic, but it has secondary hydroxyl end groups, so it lacks the desired high reactivity of poly(tetrahydrofuran) or poly(ethylene oxide).
A valuable polyether would have) reactive primary hydroxyl end groups like poly(tetrahydrofuran) or poly(ethylene oxide), but would also be non-crystalline like poly(propylene oxide). One polyether with these advantages is poly(2-methyl-1,3-propanediol), also known as poly(3-methyloxetane).
Poly(2-methyl-1,3-propanediol) has primary hydroxyl end groups and is non-crystaliine because of the presence Of the methyl group. The polymer can be made by cationic ring-opening polymerization of 3-methyloxetane as Motoi et al. teach in U.S. Pat. No. 4,672,1413-Methyloxetane can be made from 2-methyl-1,3-propanediol (see, e.g., U.S. Pat. Nos. 3,006,926 and 5,081,268), but its synthesis is extremely challenging, and yields are typically quite low (20-30%). In addition, the monomer is highly reactive, so it must be handled with care. Because 3-methyloxetane is not easy to make, polymers made from it have not become commercially available.
Even if an economical way to synthesize 3-methyloxetane could be developed, its polymerization has some disadvantages. First, because 3-methyloxetane polymerizes so rapidly, it is hard to limit the degree of polymerization to make low-molecular-weight oligomers that have potential value as reactive diluents or chain extenders for polyurethanes, or as diol components for unsaturated polyester resin and thermoplastic polyester synthesis. Usually, molecular weights will be greater than about 1000, although Motoi et al. do report a polymer having degree of polymerization=4 (see Example 1 of U.S. Pat. No. 4,672,141). In addition, the reaction is quite exothermic, which makes heat removal from the reactor an issue.
In sum, a process that would allow an economical synthesis of poly(2-methyl-1,3-propanediol) without requiring preparation or polymerization of 3-methyloxetane would be valuable.
One potential route to polyethers is glycol dehydration. Usually, however, ring-opening polymerization is a more economical and feasible approach. Poly(tetrahydrofuran), for example, is readily made by polymerizing tetrahydrofuran with strong acid catalysts. This polymer cannot be made by dehydrating 1,4-butanediol because cyclization to give tetrahydrofuran is much more favorable. Even 1,6-diols cyclize to form 5-membered cyclic ethers in the presence of an acid catalyst (e.g., 1,6-hexanediol gives 2-ethyltetrahydrofuran). Dehydration of 1,2-diols tends to give dioxanes rather than the desired polyether. In sum, glycol dehydration has not been widely used for making polyethers.
A simple way to make poly(2-methyl-1,3-propanediol), a polymer having low crystallinity and primary hydroxyl end groups, is needed. Preferably, the process would overcome the need to prepare and polymerize 3-methyloxetane. A particularly valuable process would use simple catalysts and ordinary reaction conditions. A valuable process would provide a way to limit the molecular weight of the poly(2-methyl-1,3-propanediol) to give dimers and trimers useful as reactive diluents or chain extenders for polyurethanes, and as components for unsaturated polyester resins and thermoplastic polyesters.